dm-pcache: zero cache_info before default init

dm-pcache: zero cache_info before default init

pcache_meta_find_latest() leaves whatever it last copied into the
caller’s buffer even when it returns NULL. For cache_info_init(),
that meant cache->cache_info could still contain CRC-bad garbage when
no valid metadata exists, leading later initialization paths to read
bogus flags.

Explicitly memset cache->cache_info in cache_info_init_default()
so new-cache paths start from a clean slate. The default sequence
number assignment becomes redundant with this reset, so it drops out.

Signed-off-by: Li Chen <chenl311@chinatelecom.cn>
Reviewed-by: Zheng Gu <cengku@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>